diff --git a/src/DiagrammaticEquations.jl b/src/DiagrammaticEquations.jl
index f10bfec..9529578 100644
--- a/src/DiagrammaticEquations.jl
+++ b/src/DiagrammaticEquations.jl
@@ -14,7 +14,7 @@ Collage, collate,
 oapply, unique_by, unique_by!, OpenSummationDecapodeOb, OpenSummationDecapode, Open, default_composition_diagram,
 ## acset
 SchDecapode, SchNamedDecapode, AbstractDecapode, AbstractNamedDecapode, NamedDecapode, SummationDecapode,
-contract_operators!, contract_operators, add_constant!, add_parameter, fill_names!, dot_rename!, expand_operators, infer_state_names, recognize_types,
+contract_operators!, contract_operators, add_constant!, add_parameter, fill_names!, dot_rename!, expand_operators, infer_state_names, infer_terminal_names, recognize_types,
 resolve_overloads!, replace_names!,
 apply_inference_rule_op1!, apply_inference_rule_op2!,
 transfer_parents!, transfer_children!,
diff --git a/src/acset.jl b/src/acset.jl
index c1e924b..15742cc 100644
--- a/src/acset.jl
+++ b/src/acset.jl
@@ -202,7 +202,7 @@ end
 """    function infer_states(d::SummationDecapode)
 
 Find variables which have a time derivative or are not the source of a computation.
-See also: [`infer_state_names`](@ref).
+See also: [`infer_terminals`](@ref).
 """
 function infer_states(d::SummationDecapode)
   parentless = filter(parts(d, :Var)) do v
@@ -226,7 +226,7 @@ infer_state_names(d) = d[infer_states(d), :name]
 
 """    function infer_terminals(d::SummationDecapode)
 Find variables which have no children.
-See also: [`infer_state_names`](@ref).
+See also: [`infer_states`](@ref).
 """
 function infer_terminals(d::SummationDecapode)
   filter(parts(d, :Var)) do v
@@ -237,6 +237,13 @@ function infer_terminals(d::SummationDecapode)
   end
 end
 
+"""    function infer_terminal_names(d)
+
+Find names of variables which have no children.
+See also: [`infer_terminals`](@ref).
+"""
+infer_terminal_names(d) = d[infer_terminals(d), :name]
+
 """    function expand_operators(d::SummationDecapode)
 
 Find operations that are compositions, and expand them with intermediate variables.
diff --git a/src/composition.jl b/src/composition.jl
index cc6f802..4170379 100644
--- a/src/composition.jl
+++ b/src/composition.jl
@@ -220,6 +220,13 @@ oapply(r::RelationDiagram, pode::OpenSummationDecapode) = oapply(r, [pode])
 # Default composition
 # -------------------
 
+# This helper function finds elements which appear in an array more than once.
+function find_duplicates(vs::Vector{T}) where T
+  once, twice = Set{T}(), Set{T}()
+  foreach(v -> v ∈ once ? push!(twice,v) : push!(once,v), vs)
+  twice
+end
+
 # TODO: Upstream this to Catlab?
 function construct_relation_diagram(boxes::Vector{Symbol}, junctions::Vector{Vector{Symbol}})
   tables = map(boxes, junctions) do b, j
@@ -246,11 +253,11 @@ function default_composition_diagram(podes::Vector{D}, names::Vector{Symbol}, on
     pode[findall(!=(:Literal), pode[:type]), :name]
   end
   for (nln, name) in zip(non_lit_names, names)
-    allunique(nln) || error("Decapode $name contains repeated variable names: $nln.")
+    allunique(nln) || error("Decapode $name contains repeated variable names: $(find_duplicates(nln)).")
   end
   if only_states_terminals
     foreach(non_lit_names, podes) do nln, pode
-      outers = infer_state_names(pode) ∪ pode[infer_terminals(pode), :name]
+      outers = infer_state_names(pode) ∪ infer_terminal_names(pode)
       filter!(x -> x ∈ outers, nln)
     end
   end